Rails ActiveSupport comes with a number of classes to make date/time management in Ruby more flexible. The 'duck typing' conventions of Ruby programming usually make it irrelevant whether a time object is an instance of type Time or a class that supports the same methods.

The BSON conversion within the mongo-ruby driver breaks these conventions. This creates particular difficulties for Rails developers who work with the config.time_zone parameter set, which causes most times to be cast into ActiveSupport::TimeWithZone. This class, along with Date and DateTime and others, presents an exception "Mongo::InvalidDocument: Unsupported type for BSON (1)".

My suggestion would be to implement more testing in the bson_type method. In addition to the === Time test that's already in the case statement, you could cast to Time when the object responds to 'to_time'. Other types could be similarly tested for their to_* equivalents as well. If that seems too murky, putting class checks against the common extension classes in the Ruby standard library and in ActiveSupport would be a good 80% solution.